Laser discharge chamber passivation by plasma

ABSTRACT

Methods and apparatus are provided for cleaning and passivating laser discharge chambers with plasmas. In one embodiment, an oxygen based plasma is formed in a plasma source external to the laser discharge chamber by applying a radiofrequency signal to oxygen containing gases. The oxygen based plasma is drawn into the laser discharge chamber, where it reacts with contaminants and cleans internal surfaces. After cleaning, a fluorine based plasma is formed in the plasma source and drawn into the laser discharge chamber to passivate internal surfaces. In another embodiment, cleaning with the oxygen based plasma is not used since some level of cleaning is accomplished with the fluorine based plasma. In another embodiment, oxygen based plasmas and fluorine based plasmas are formed in the laser discharge chamber by applying a radiofrequency signal to a laser discharge chamber electrode. Plasma cleaning and passivation of laser discharge chambers is safer, more efficient, and more effective than conventional thermal cleaning and passivation processes.

BACKGROUND

[0001] 1. Field of the Invention

[0002] The invention relates to methods and apparatus for cleaning andpassivating laser discharge chambers. More particularly, the inventionrelates to methods and apparatus for cleaning and passivating laserdischarge chambers utilizing plasmas.

[0003] 2. Description of the Related Art

[0004] Gas lasers in which the lasing medium includes fluorine orfluorine compounds are the workhorse light sources for the integratedcircuit lithography industry. In fluorine based gas lasers such askrypton fluoride (KrF) excimer lasers, argon fluoride (ArF) excimerlasers, and molecular fluorine (F₂) lasers, a high energy electricaldischarge excites a gas mixture in a discharge chamber to produce aplasma which serves as the lasing medium.

[0005] The performance of such lasers is degraded by the presence ofimpurities in the discharge chamber inadvertently introduced ascontaminants during the manufacturing process, introduced by exposure tothe ambient environment, or produced by reactions between the gasmixture and contaminants or chamber materials. Such impurities includeHF, CF₄, COF₂, SiF₄, CO₂, various hydrocarbons, and H₂O. Impurities candegrade the profile of the laser beam by fouling optical components,reduce the lifetime of the gas fill by reacting with and consuming thegas mixture, and reduce output power by absorbing laser light and byquenching the excited species such as ArF, KrF, and F₂ that supportlasing. Also, highly reactive impurities such as HF corrode the internalsurfaces of the discharge chamber. Impurities are detrimental even atlow concentrations. For example, it has been observed that the presenceof CO₂ at concentrations as low as 30 parts per million in a KrF excimerlaser plasma can reduce the output power of the laser by 5%.Consequently, the discharge chamber must be cleaned of impurities.

[0006] The plasma lasing medium includes highly reactive fluorinespecies which also corrode unprotected internal surfaces of thedischarge chamber. Consequently, the materials in the discharge chambermust be passivated to protect them from the plasma lasing medium.

[0007] Laser discharge chambers for fluorine based gas lasers areconventionally cleaned and passivated with a thermal process such as thefollowing. A discharge chamber is heated to approximately 100° C. andevacuated with a vacuum pump to a pressure of approximately 20millitorr. This temperature and pressure is maintained for at least 8hours, during which some of the volatile contaminants in the dischargechamber, such as water, are removed by the pump. The discharge chamberis then filled with a mixture of approximately 5% F₂ and approximately95% helium, neon, or other inert gas at a pressure of approximately oneatmosphere. The temperature is maintained at 100° C. for at least 4hours, during which a fluorine based passivation layer forms on some ofthe internal surfaces of the discharge chamber.

[0008] The fluorine gas used in conventional processes poses a safetyrisk, as it is highly corrosive and highly toxic. Also, the conventionalprocess is not entirely effective. The discharge chamber must undergo asubsequent 24 hour burn-in operation period, during which the gasmixture is replaced multiple times, before laser operation issatisfactory. Furthermore, the conventional process requires at least 12hours, more typically 24 to 48 hours, and is therefore inefficient.

[0009] What is needed is a laser discharge chamber cleaning andpassivation process that is safer, more effective, and more efficientthan conventional cleaning and passivation methods.

SUMMARY

[0010] Methods and apparatus are provided for cleaning and passivatinglaser discharge chambers with plasmas. In one embodiment, an oxygenbased plasma is formed in an external plasma source by inductivelyapplying a radiofrequency signal to oxygen containing gases such as O₂,N₂O, and mixtures thereof. The oxygen based plasma is drawn into thelaser discharge chamber, where it reacts with contaminants to producevolatile species which are removed by a vacuum pump, thereby cleaningthe laser discharge chamber.

[0011] After the oxygen based plasma cleaning process, a fluorine basedplasma is formed in the external plasma source by inductively excitingfluorine containing gases such as NF₃, F₂, CF₄, SF₆, and mixturesthereof with a radiofrequency signal. The fluorine based plasma is drawninto the laser discharge chamber, where it reacts with internal surfacesto form a protective passivation layer. The fluorine based plasma alsoreacts with contaminants to produce volatile species which are removedby the vacuum pump. Internal surfaces of the laser discharge chamber arethereby cleaned and passivated. Since the fluorine based plasma alsocleans the chambers, the oxygen based plasma cleaning process isoptional.

[0012] In another embodiment, oxygen based plasma and fluorine basedplasmas are formed in the laser discharge chamber by applying aradiofrequency signal to a laser discharge chamber electrode and therebyexciting oxygen containing gases and fluorine containing gases. Theoxygen and fluorine based plasmas react with contaminants and withinternal surfaces to clean and passivate the laser discharge chamber.

[0013] Plasma cleaning and passivation of laser discharge chambers doesnot require the use of dangerous F₂ gas. Also, plasma cleaning andpassivation is much less time consuming, and thus much more efficient,than conventional thermal cleaning and passivation processes. Moreover,laser discharge chambers cleaned and passivated with plasmas exhibitimproved performance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic diagram of an apparatus including anexternal plasma source for cleaning and passivating a laser dischargechamber in accordance with one embodiment of the present invention.

[0015]FIG. 2 is a schematic diagram of an apparatus for cleaning andpassivating a laser discharge chamber with internally generated plasmasin accordance with one embodiment of the present invention.

[0016]FIG. 3 is plot comparing the performance of laser dischargechambers passivated according to the present invention to laserdischarge chambers passivated by conventional methods.

DETAILED DESCRIPTION

[0017]FIG. 1 is a schematic diagram of an apparatus for cleaning andpassivating a laser discharge chamber 2 with plasmas according to oneembodiment of the present invention. Laser discharge chamber 2 may be,for example, a discharge chamber for a KrF excimer laser such as themodel 5000 KrF excimer laser manufactured by Cymer, Incorporated. Themodel 5000 discharge chamber has a volume of about 20 liters andinternal surfaces of brass, electroless nickel plated aluminum, andceramic. Laser discharge chamber electrode 3, inside laser dischargechamber 2, is typically a conducting metal or metal alloy such as brass.Other laser discharge chambers, such as discharge chambers for ArFexcimer lasers and discharge chambers for molecular fluorine (F₂)lasers, may also be cleaned and passivated in accordance with thepresent invention.

[0018] Laser discharge chamber 2 is placed in contact with a heater 4,which heats discharge chamber 2 to an elevated temperature, in oneembodiment about 70° C., thereby driving volatile contaminants frominternal surfaces of discharge chamber 2 and also facilitatingsubsequent passivating chemical reactions on those surfaces. Heater 4may be an electrically heated metal plate on which discharge chamber 2is placed and enclosed with a cover, for example. The present inventionis independent of the type of heater used. In one embodiment, thetemperature of discharge chamber 2 settles at about 70° C. after about45 minutes of heating.

[0019] After being installed on heater 4, laser discharge chamber 2 isconnected to a purge gas line 6 at a discharge chamber valve 8, to anexternal plasma source 10 at a discharge chamber window assembly 12, andto a vacuum line 14 at a discharge chamber window assembly 16.Advantageously, with this geometry plasmas may be drawn the length ofdischarge chamber 2 to uniformly clean and passivate internal dischargechamber surfaces. Of course, depending on the design of the chamber,connections to purge gas line 6, plasma source 10, and vacuum line 14may be made in many other geometries and might utilize chamber windows,valves, and ports not shown in FIG. 1.

[0020] All gas lines, vacuum lines, and valves are made from high puritystainless steel to minimize introduction of contaminants into dischargechamber 2. Internal seals in valves and flanges are preferably made offluorine-resistant metal. Seals made of fluorine resistant perfluoroelastomers such as Kalrez® are also acceptable, however.

[0021] In one embodiment, external plasma source 10 is a Delta Glow™ DG300 or DG 600 inductively coupled high energy plasma source manufacturedby Manitou Systems, Incorporated. Advantageously, the antenna of such aninductively coupled plasma source is not in physical contact with theplasma and thus does not introduce contaminants into discharge chamber 2during the cleaning and passivating process. A quartz glass reactor tubein the Delta Glow™ plasma source is replaced with a 99.8% purity alumina(Al₂O₃) tube to prevent etching of the reactor tube by fluorine basedplasmas. Other external plasma sources, inductively or directly coupled,are used in alternative embodiments.

[0022] Next, programmable controller 18 opens pressure control valve 20to allow vacuum pump 22 to evacuate vacuum line 14, discharge chamber 2,and external plasma source 10. The exhaust from vacuum pump 22, which insubsequent process steps may contain fluorine compounds, is passedthrough exhaust waste gas scrubber 24 to remove corrosive or toxicexhaust constituents. The conductance through plasma source 10,discharge chamber 2, and vacuum line 14 is sufficiently high that theyare at essentially equal pressure.

[0023] In one embodiment, programmable controller 18 is a SYSMAC modelC200HG programmable logic controller manufactured by Omron Electronics,Incorporated. Alternative embodiments employ other programmablecontrollers, or manual control.

[0024] Conventional vacuum pumps employing oil are vulnerable to attackby corrosive gases employed or generated in the cleaning and passivatingprocesses. In one embodiment, vacuum pump 22 is a QDP80 dry pump with apumping capacity of 80 liters per minute manufactured by BOC Edwards,Incorporated.

[0025] Controller 18 reads capacitance manometer 26, which measures thepressure in vacuum line 14, and controls pressure control valve 20 toset the pressure in vacuum line 14, discharge chamber 2, and plasmasource 10 to about 20 millitorr. In one embodiment, capacitancemanometer 26 is a CeramiCel® capacitance manometer manufactured byVarian, Incorporated, and pressure control valve 20 is a model 651CD2S1Npressure control valve manufactured by MKS Instruments, Incorporated.Capacitance manometers provide accurate and stable absolute pressuremeasurements, facilitating reproducible cleaning and passivation.

[0026] After the pressure has reached about 20 millitorr, controller 18closes valve 20 and monitors the pressure for about 20 minutes to testfor leaks. Other conventional leak testing techniques, such as heliumleak testing, are also used. After proper installation has beenverified, controller 18 reopens valve 20 and vacuum pump 22 again pullsthe pressure down to about 20 millitorr.

[0027] In one embodiment, discharge chamber 2 is next cleaned with anoxygen based plasma formed in plasma source 10 from gases includingoxygen containing gases such as oxygen (O₂), N₂O, and mixtures thereof,prior to being cleaned and passivated with a fluorine based plasmaformed in plasma source 10 from gases including fluorine containinggases such as NF₃, CF₄, F₂, SF₆, freons, and mixtures thereof. Thefluorine based plasma cleaning and passivation process may be followedby additional oxygen based plasma and fluorine based plasma cleaning andpassivation processes. In another embodiment, oxygen based plasmacleaning of discharge chamber 2 is not utilized, and discharge chamber 2is next cleaned and passivated with a fluorine based plasma. In anotherembodiment, discharge chamber 2 is cleaned and passivated with an oxygenand fluorine based plasma formed from a mixture of oxygen containing andfluorine containing gases.

[0028] In one embodiment, for example, controller 18 opens valves 28 and30 to allow oxygen to flow from oxygen supply 32, through mass flowcontroller (MFC) 34, external plasma source 10, discharge chamber 2, andvacuum line 14 to vacuum pump 22. MFC 34, in one embodiment a model1259C-0050GK MFC manufactured by MKS Instruments, Incorporated,regulates the flow of oxygen to a rate of typically about 10 standardcubic centimeters per minute (sccm) to about 50 10 sccm. The pressureregistered by capacitance manometer 26 rises to about 100 millitorr toabout 1.5 torr, depending on the oxygen flow rate. During an optionalpurge period of from about 1 minute to about 10 minutes duration,flowing oxygen displaces other gases in plasma source 10, dischargechamber 2, and vacuum line 14.

[0029] After the optional purge period, and with oxygen continuing toflow through plasma source 10, discharge chamber 2, and vacuum line 14,controller 18 turns on radio frequency (RF) power supply 36. Powersupply 36 delivers about 100 Watts to about 600 Watts, in one embodimentabout 400 Watts, of 13.56 MHz continuous RF power through impedancematching network 38 to external plasma 20 source 10. The RF powerexcites the oxygen in plasma source 10 to form an oxygen plasmacontaining reactive oxygen ions and radicals, which flows from plasmasource 10 into and through discharge chamber 2. The oxygen plasmaoxidizes hydrocarbon contaminants in discharge chamber 2 to producevolatile reaction products, such as CO₂ and H₂O, which are removed byvacuum pump 22. 25 Internal surfaces of discharge chamber 2 are therebycleaned of contaminants.

[0030] Since the plasma is excited externally, all internal surfaces ofdischarge chamber 2 are at equal electrical potential. Advantageously,externally generated plasmas consequently interact uniformly with theinternal surfaces of discharge chamber 2.

[0031] In one embodiment, RF power supply 36 is a model MS600 powersupply manufactured by Manitou Systems, Incorporated capable ofdelivering up to about 600 Watts at about 13.56 MHz in pulsed orcontinuous mode to plasma source 10. Though 13.56 MHz is an industrystandard, other radio frequencies can also be used. In an alternativeembodiment, RF power supply 36 is a model CESAR 136 power supplymanufactured by Dressler HF-Technik GmbH. Impedance matching network 38,in one embodiment a model RFS-1004 automatic impedance matching networkmanufactured by RF Services, Incorporated, maximizes power transfer fromRF power supply 36 to the plasma load. In an alternative embodiment,impedance matching network 38 is a model ATR impedance matching networkmanufactured by Manitou Systems, Incorporated.

[0032] An oxygen plasma is formed in plasma source 10 and drawn throughdischarge chamber 2 for a period of about 0.5 hours to about 2.0 hours,depending upon the RF power used. Higher RF powers produce a morereactive oxygen plasma, which requires less time to clean dischargechamber 2. In one embodiment, the period of exposure to the oxygenplasma is a predetermined period known by experiment to be sufficientlylong for the oxygen plasma to satisfactorily clean discharge chamber 2.

[0033] In alternative embodiments, exposure to the oxygen plasmacontinues until an endpoint of the cleaning process is detected. Anendpoint may be defined by concentrations of one or more chemicalspecies in the plasma or exhaust gas reaching particular values. Forexample, the concentration of CO₂ in the gas flowing out of dischargechamber 2 decreases as hydrocarbon contaminants are depleted. Thus, anendpoint may be defined by the concentration of CO₂ decreasing to reacha particular value which indicates that the chamber is sufficientlyclean.

[0034] In one embodiment, an endpoint is determined by residual gasanalyzer (RGA) 40, which is a model TSPTC100(2100) RGA manufactured byLeybold Inficon, Incorporated. RGA 40, which is in communication withcomputer 42 via a conventional RS-232 interface, monitors theconcentrations of the various chemical species present in vacuum line14. In another embodiment, an endpoint is determined by optical monitor44. Optical monitor 44 excites chemical species present in vacuum line14 with an electrical discharge, and measures their optical emission tomonitor their concentrations. In another embodiment, an endpoint isdetermined by monitoring the RF power reflected from plasma source 14.The reflected RF power is known in the art to characterize the plasma.

[0035] At the end of the predetermined period of exposure to the oxygenplasma, or when an endpoint is detected, controller 18 closes valves 28and 30 and turns off RF power supply 36. Vacuum pump 22 pulls thepressure down to about 20 millitorr.

[0036] Next, discharge chamber 2 is cleaned and passivated with afluorine based plasma formed in plasma source 10. In one embodiment, forexample, controller 18 opens valves 28 and 46 to allow NF₃ to flow fromNF₃ supply 48, through MFC 34, external plasma source 10, dischargechamber 2, and vacuum line 14 to vacuum pump 22. MFC 34 regulates theflow of NF₃ to a rate of typically about 5 sccm to about 25 sccm. Thepressure rises to about 100 millitorr to about 1.5 torr, depending onthe NF₃ flow rate. During an optional purge period of from about 1minute to about 10 minutes duration, flowing NF₃ displaces other gasesin plasma source 10, discharge chamber 2, and vacuum line 14.

[0037] After the optional purge period, and with NF₃ continuing to flowthrough plasma source 10, discharge chamber 2, and vacuum line 14,controller 18 turns on RF power supply 36. Power supply 36 deliversabout 100 Watts to about 600 Watts, in one embodiment about 400 Watts,of 13.56 MHz continuous RF power through impedance matching network 38to plasma source 10. The RF power excites the NF₃ in plasma source 10 toform a fluorine based plasma which contains reactive fluorine speciessuch as F and F₂ radicals and ions and produces intense ultravioletradiation. The fluorine based plasma flows from plasma source 10 intoand through discharge chamber 2. Advantageously, high concentrations ofreactive fluorine species and intense ultraviolet radiation areintroduced into discharge chamber 2 without requiring the use of F₂ as aprecursor gas.

[0038] The fluorine based plasma reacts with contaminants in dischargechamber 2 to produce volatile reaction products, such as HF and SiF₄,which are removed by vacuum pump 22. The fluorine based plasma alsoreacts with the internal surfaces of discharge chamber 2 to formpassivating layers which protect the surfaces from further reaction withfluorine based plasmas such as fluorine based plasma lasing media. Forexample, the fluorine based plasma reacts with Nickel surfaces to formstable NiF₂ layers, with stainless steel surfaces to form stable FeF₂layers, and with alumina (Al₂O₃) surfaces to form stable AlF₃ layers.This passivation process is the primary role of the fluorine basedplasma.

[0039] The fluorine based plasma is produced in plasma source 10 anddrawn through discharge chamber 2 for a period of about 0.5 hours toabout 2.0 hours, depending upon the RF power used. As with the oxygenbased plasmas, higher RF powers produce a more reactive fluorine basedplasma, which requires less time to clean and passivate laser dischargechamber 2.

[0040] In one embodiment, the period of exposure to the fluorine basedplasma is a predetermined period chosen to be sufficiently long tosatisfactorily clean and passivate discharge chamber 2. In alternativeembodiments, exposure to the fluorine based plasma continues until anendpoint is detected with RGA 40, with optical monitor 44, or withmeasurements of reflected RF power. The endpoint may be a particularconcentration of molecular fluorine in discharge chamber 2 or vacuumline 14, for example. As the internal surfaces of discharge chamber 2are passivated, fluorine consumption decreases and the concentration ofmolecular fluorine grows to a value indicating that discharge chamber 2is sufficiently clean and passivated.

[0041] At the end of the predetermined period of exposure to thefluorine based plasma, or when an endpoint is detected, controller 18closes valves 34 and 46 and turns off RF power supply 36. Vacuum pump 22pulls the pressure down to about 20 millitorr. Discharge chamber 2 ischecked for leaks, and controller 18 closes pressure control valve 20.

[0042] Next, discharge chamber 2, plasma source 10, and vacuum line 14are back filled with an inert gas such as helium, nitrogen, neon,krypton, and mixtures thereof. In one embodiment, helium from heliumsupply 50 flows through valve 52, purge gas line 6, and valve 8 topressurize discharge chamber 2 to about 1 pound per square inch overatmospheric pressure. Under inert gas purge, which prevents ambient airfrom entering and contaminating discharge chamber 2, plasma source 10and vacuum line 14 are disconnected from discharge chamber 2, and windowassemblies 12 and 16 are sealed. Valve 8 is closed, gas line 6 isdisconnected, and discharge chamber 2 is removed from heater 4.

[0043] Laser discharge chamber 2 may also be cleaned and passivated withinternally generated plasmas. FIG. 2 is a schematic diagram of anapparatus for cleaning and passivating a laser discharge chamber 2 withinternally generated plasmas in accordance with one embodiment of thepresent invention. Like numbers in FIG. 1 and FIG. 2 designate the sameparts in the various embodiments.

[0044] Processes for cleaning and passivating discharge chamber 2 withinternally generated plasmas differ from the processes described aboveutilizing externally generated plasmas primarily in the delivery ofprecursor gases to discharge chamber 2, and in the coupling of RF powerto the plasma. Other process steps and parameters are substantially thesame as those described above.

[0045] Oxygen and fluorine containing gases, such as those listed above,flow through gas line 7, valve 8, discharge chamber 2, and vacuum line14 to vacuum pump 22. RF power supply 36 delivers RF power throughimpedance matching network 38 to discharge chamber electrode 3.Discharge chamber electrode 3 is used in normal laser operation ofdischarge chamber 2 to generate a fluorine based plasma lasing medium.Here, discharge chamber electrode 3 is used as an RF antenna. The RFpower delivered to discharge chamber electrode 3 excites the gases toform oxygen based plasmas and fluorine based plasmas containing reactivechemical species which clean and passivate the internal surfaces ofdischarge chamber 2. Discharge chamber 2 is subsequently purged withhelium delivered through gas line 7 and valve 8.

[0046] Advantageously, passivation of laser discharge chambers withplasmas in accordance with the present invention requires only about 2to 4 hours, rather than the 24 to 48 hours required by conventionalpassivation processes. Moreover, the performance of lasers withdischarge chambers passivated in accordance with the present inventioncompares favorably to that of lasers with discharge chambers passivatedby conventional methods.

[0047] The performance of KrF excimer lasers was evaluated by measuringthe normal operation discharge voltage during a series of test periodsfollowing initial turn-on of the laser. Lower discharge voltagesindicate less fluorine consumption by internal surfaces of dischargechamber 2 during normal operation of the laser, and thus betterperformance. FIG. 3 is a plot of the normal operation discharge voltage(HV) versus test number for an average of 88 discharge chamberspassivated with a conventional thermal process (diamonds), and for anaverage of about 10 chambers passivated with plasmas in accordance withthe present invention (squares). The plasma passivated chambers werefirst cleaned for about 2 hours with an externally generated oxygenplasma, and then cleaned and passivated for about 2 hours with anexternally generated fluorine based plasma. As FIG. 3 indicates, theperformance of the plasma passivated chambers is as much as one standarddeviation (sigma) better than that of the conventionally passivatedchambers.

[0048] While the present invention is illustrated with particularembodiments, the invention is intended to include all variations andmodifications falling within the scope of the appended claims.

We claim:
 1. A method of passivating a laser discharge chamber,comprising: forming a fluorine based plasma from one or more firstgases, said first gases comprising a fluorine containing gas; andreacting said fluorine based plasma with internal surfaces of said laserdischarge chamber to passivate said internal surfaces.
 2. The method ofclaim 1 wherein said fluorine containing gas is a fluorine containinggas selected from the group consisting of NF₃, F₂, CF₄, SF₆, andmixtures thereof.
 3. The method of claim 1 wherein forming said fluorinebased plasma comprises: flowing said first gases into a plasma sourceexternal to said laser discharge chamber, said fluorine containing gasflowing at a flow rate of about 5 standard cubic centimeters per minute(sccm ) to about 25 sccm; applying a radio frequency signal to saidfirst gases to form said fluorine based plasma in said plasma source;and flowing said fluorine based plasma into said laser dischargechamber.
 4. The method of claim 3 wherein applying said radio frequencysignal to said first gases comprises inductively applying said radiofrequency signal to said first gases.
 5. The method of claim 1 whereinforming said fluorine based plasma comprises: flowing said first gasesinto said laser discharge chamber, said fluorine containing gas flowingat a flow rate of about 5 sccm to about 25 sccm; and applying a radiofrequency signal to said first gases to form said fluorine based plasmain said laser discharge chamber.
 6. The method of claim 1 whereinreacting said fluorine based plasma with internal surfaces of said laserdischarge chamber comprises reacting said fluorine based plasma withsaid internal surfaces for a first period of time of about 0.5 hours toabout 2.0 hours.
 7. The method of claim 6 further comprising selectingsaid first period of time by determining a fluorine plasma reactionendpoint.
 8. The method of claim 6 further comprising maintaining apressure in said laser discharge chamber of about 100 millitorr to about1.5 torr for said first period of time.
 9. The method of claim 1 furthercomprising maintaining a temperature of said discharge chamber of about50° C. to about 100° C.
 10. The method of claim 1 further comprisingflowing said fluorine based plasma through said laser discharge chamberand out of a discharge chamber exit port.
 11. The method of claim 1wherein said first gases comprise an oxygen containing gas.
 12. Themethod of claim 1 further comprising: forming an oxygen based plasmafrom one or more second gases, said second gases comprising an oxygencontaining gas; and reacting said oxygen based plasma with said internalsurfaces of said laser discharge chamber to clean said internalsurfaces.
 13. The method of claim 12 wherein said oxygen containing gasis an oxygen containing gas selected from the group consisting of O₂,N₂O, and mixtures thereof.
 14. The method of claim 12 wherein formingsaid oxygen based plasma comprises: flowing said second gases into aplasma source external to said laser discharge chamber, said oxygencontaining gas flowing at a flow rate of about 10 sccm to about 50 sccm;applying a radio frequency signal to said second gases to form saidoxygen based plasma in said plasma source; and flowing said oxygen basedplasma into said laser discharge chamber.
 15. The method of claim 14wherein applying said radio frequency signal to said second gasescomprises inductively applying said radio frequency signal to saidsecond gases.
 16. The method of claim 12 wherein forming said oxygenbased plasma comprises: flowing said second gases into said laserdischarge chamber, said oxygen containing gas flowing at a flow rate ofabout 10 sccm to about 50 sccm; and applying a radio frequency signal tosaid second gases to form said oxygen based plasma in said laserdischarge chamber.
 17. The method of claim 12 wherein reacting saidoxygen based plasma with internal surfaces of said laser dischargechamber comprises reacting said oxygen based plasma with said internalsurfaces for a second period of time of about 0.5 hours to about 2.0hours.
 18. The method of claim 17 further comprising selecting saidsecond period of time by determining an oxygen plasma reaction endpoint.19. The method of claim 17 further comprising maintaining a pressure insaid laser discharge chamber of about 100 millitorr to about 1.5 torrfor said second period of time.
 20. The method of claim 12 furthercomprising flowing said oxygen based plasma through said laser dischargechamber and out of a discharge chamber exit port.
 21. An apparatus forpassivating a laser discharge chamber, comprising: a source of one ormore gases, said source of gases coupled to said laser dischargechamber, said gases comprising a fluorine containing gas; a source of aradiofrequency signal; an antenna electrically coupled to said source ofa radiofrequency signal, whereby said radiofrequency signal is appliedto said gases to form a plasma.
 22. The apparatus of claim 21 whereinsaid fluorine containing gas is a fluorine containing gas selected fromthe group consisting of NF₃, F₂, CF₄, SF₆, and mixtures thereof.
 23. Theapparatus of claim 21 wherein said gases comprise an oxygen containinggas.
 24. The apparatus of claim 23 wherein said oxygen containing gas isan oxygen containing gas selected from the group consisting of O₂, N₂O,and mixtures thereof.
 25. The apparatus of claim 21 wherein saidradiofrequency signal is of a frequency of about 13.56 MHz and of apower of about 100 Watts to about 600 Watts.
 26. The apparatus of claim21 wherein said antenna is a laser discharge chamber electrode internalto said laser discharge chamber.
 27. The apparatus of claim 21 furthercomprising a plasma source external to said laser discharge chamber,said plasma source coupled to said source of gases and coupled to saidlaser discharge chamber, said antenna internal to said plasma source.28. The apparatus of claim 27 wherein said antenna is inductivelycoupled to said gases.
 29. The apparatus of claim 21 further comprisinga mass flow controller coupled to said source of gases and coupled tosaid laser discharge chamber, whereby a flow rate of said gases isregulated.
 30. The apparatus of claim 21 further comprising a pressurecontrol valve, a pressure gauge, and a vacuum pump, said pressurecontrol valve coupled to said laser discharge chamber and coupled tosaid vacuum pump, said pressure gauge coupled to said laser dischargechamber.
 31. The apparatus of claim 30 further comprising a controllerfor controlling said source of a radiofrequency signal, said source ofgases, and said pressure control valve.
 32. The apparatus of claim 21further comprising a heater in contact with said laser dischargechamber.
 33. The apparatus of claim 21 further comprising a residual gasanalyzer coupled to said laser discharge chamber, whereby a plasmareaction endpoint is determined.
 34. The apparatus of claim 21 furthercomprising an optical monitor coupled to said laser discharge chamber,whereby a plasma reaction endpoint is determined.
 35. The apparatus ofclaim 21 further comprising an impedance matching network electricallycoupled to said source of a radiofrequency signal, and electricallycoupled to said antenna.